The institute of electrical and electronics engineers (IEEE) 802.16 standard provides a technique and protocol for supporting broadband wireless access. The standardization had been conducted since 1999 until the IEEE 802.16-2001 was approved in 2001. The IEEE 802.16-2001 is based on a physical layer of a single carrier (SC) called ‘WirelessMAN-SC’. The IEEE 802.16a standard was approved in 2003. In the IEEE 802.16a standard, ‘WirelessMAN-OFDM’ and ‘WirelessMAN-OFDMA’ are further added to the physical layer in addition to the ‘WirelessMAN-SC’. After completion of the IEEE 802.16a standard, the revised IEEE 802.16-2004 standard was approved in 2004. To correct bugs and errors of the IEEE 802.16-2004 standard, the IEEE 802.16-2004/Cor1 was completed in 2005 in a format of ‘corrigendum’. In addition, IEEE 802.16e (hereinafter, ‘802.16e’) was approved in December, 2005.
At present, there is an ongoing effort to complete the IEEE 802.16m-“Air Interface for Fixed and Mobile Broadband Wireless Access Systems-Advanced Air Interface” standard (hereinafter, ‘802.16m’) by 2010 as a new technical standard on the basis of the existing 802.16e. The newly developed technical standard, i.e., 802.16m, has to be designed so that the existing 802.16e can be also supported. That is, the technique (i.e., 802.16m) of a newly designed system has to be configured such that the system operates by effectively integrating the existing technique (i.e., 802.16e) (in terms of backward compatibility).
A multi-carrier mode is supported in the 802.16m. Herein, the support of the multi-carrier mode implies that a user equipment operating at a narrow band (e.g., 5 MHz) and a user equipment operating at a wide band (e.g., 20 MHz) are both supported. If one carrier frequency band is 5 MHz for example in the multi-carrier mode, four carriers can be combined to allocate a frequency band of 20 MHz to the user equipment operating at the wide band, thereby being able to support the user equipment operating at the wide band.
A plurality of carriers may exist in a frequency domain, and in order to prevent inter-carrier interference, a guard band exists in an edge portion of each carrier (in the frequency domain). In the existing 802.16e, the guard band was an empty band in which no data is transmitted. However, in a case where the multi-carrier mode is supported in the 802.16m, the guard band can be used for data transmission, and thus a signal for channel estimation has to be transmitted also in the guard band.
A midamble is a signal used for estimating a channel state. For example, in order to estimate a channel state for each antenna in a multiple-input multiple-output (MIMO) system which uses a plurality of antennas, a base station transmits a MIMO midamble. A user equipment receives the MIMO midamble to estimate the channel state for each antenna of the BS, and uses this estimation value to perform band selection, antenna selection, precoding matrix index determination, etc.
Accordingly, a method for transmitting a midamble in a multi-carrier mode is required.